Just a quick list, off the top of my head:
1. Any adventure that interests you, any edition (any rule set). Grab 'em. With a bit of creativity, they're all convertible. As I've said before, I've had some success lately with the DCC RPG adventures (highly recommend many of em).
2. World Builder's Guidebook (2e) plus the Dungeon Builder's Guidebook (2e) are staples for idea generation.
3. Campaign Setting Boxed Sets are always a great haul if you can find them. Lots of ideas, and rules usually have a conversion floating around the internet somewhere.
4. Any of the 2e "Green Books" (Historical Supplements) are good, EXCEPT "A mighty fortress", which reads like a first year history paper (I get that philosophy and politics are important for history. I have a degree in history. But I don't need a whole chapter on it for a swashbuckling setting!). The Rome one is my personal favourite, and is actually pretty good (though the city of Rome could use more detail)
5. The 4e setting books are great. The adventure locales, like Hammerfast and Vor Rukoth. Lots of ideas, maps, and a few good NPCs (with hooks in abundance!). I'm also a HUGE fan of the setting book (but not the adventure) for the Scepter Tower of Spelgard.
6. Avoid the brown "Complete" books for 2e. They're mostly bad, though I did like chunks of the wizard one and the druid one.
7. If you're GM'ing a lot, you can get success with monster books, but I find they're generally pretty blah in the TSR days (and that includes anything post MM1, for you all you grognards! For every abishai, there are three flail snails, or stats for giant fish that Gary caught over the weekend at Lake Geneva).
8. Unearthed Arcana for 3e is pretty much my gaming Bible.
9. Also, the BECMI rules Cyclopedia. Best gaming book ever. My girlfriend is on the lookout for a decent quality one on amazon that's less than a hundred bucks. I think it's my birfday present (fingers crossed)
10. Avoid Battlesystem, or anything that is just a "rules expansion", unless the topic is something you're willing to do rules work on anyways and just want ideas. So, no skills and powers, combat and tactics, spells and magic, or any of those later 2e releases. Or Miniatures Handbook, Book of Nine Swords, etc. Basically, it's buying homework.
11. Dungeon Magazine is the best bang for your buck. I find it telling that if you go on Amazon, issues of Dungeon often go for at least five or six bucks each, whereas the issues of Dragon are typically two bucks or so. The adventures in Dungeon are usually pretty good (though there are a lot of flail snails too, I'll admit). If you find them, grab them. They get top priority when I hit used book stores.
12. Generally, avoid BECMI rules books (except the Rules Cyclopedia, which is a lot more than Rules!). But the Gazeteer books are sometimes great. "The Northern Reaches" is a personal favourite. Basically, most are worth getting, EXCEPT for the Ierendi one (I think - I'm thinking of the one that's written like a tourism pamphlet). Also, I think "Orcs of Thar" was a dud, too.
13. The various "Book of Lairs" series in 1e and 2e are great finds. Lots of small adventures that will take an evening to run through. Great for a GM forced to "wing it" and needs some sort of framework to run from. The 4e "Dungeon Delve" is a more modern version of the idea, and while it gets some flak, I think does its job admirably and is one of my favourite (and most-used) 4e releases.
14. Speaking of 4e, the planes book and a few of the creatures books offer some great fluff. The Astral Sea book is a great read, and the rules material is fairly light. It has less of the gonzo-y 4e stuff than we usually see.... or rather, it's still there, but since it's set in the Astral Plane, it's easier to accept, as opposed to "Um, it's like Medieval England... but with LASER SWORDS AND ELF LORDS AND, AND... WEREWOLF KNIGHTS!"
(Actually, I'd totally play that. Shotgun on the Werebear Landsknecht).
15. Finally, keep an eye on non-D&D purchases when used book shopping. Earthdawn, Savage Worlds, Pathfinder (obviously), Rolemaster, Warhammer Fantasy RPG, DCC RPG... all can be used for idea mining with a bit of work.
